I have a couple of comments about Boost.Predef: Undefined vs null ================= You define macros for the pertinent platform (OS, CPU, compiler, standard) and set their value to the appropriate version number. If no version number could be determined, 0.0.1 is used instead. But you also define macros for all non-pertinent platforms and set their value to 0.0.0. I know that you are doing this to allow the compiler (rather than the preprocessor) to use the macros. However, this design decision makes the by far most common use case (#ifdef MACRO) more difficult in order to support a rare use case (if (MACRO)). This is not how developers are used to working with platform macros, and it will be an eternal source of confusion. I encourage you to reconsider this design decision, and instead only define macros that are relevant for the platform. This is more in line with the expectations of your future users. Version layout ============== You encode the version number as decimal numbers in the range from 0 to 1000000000. If you encode them as hexadecimal numbers instead, you can utilize the full range from 0 to 2^32. Extension macros ================ Some macros, such as __GNUC__, _MSC_VER, and _WIN32 serve a dual purpose. They are partly used to identify a platform, and partly to enable certain extensions in header files. Other compilers will therefore define these macros as well when they want access to these extensions. For instance clang defines __GNUC__ (and related macros). If you wish BOOST_CXX_GNUC only to be defined for GCC, then you need to exclude all other compilers that may set the __GNUC__ macro. For example: #if defined(__GNUC__) && !defined(__clang__) As your predef/compiler/gcc.h file has to know the clang macro too, you might as well put all compiler macros into a single file (as discussed earlier in the thread.) C++ version =========== Your choice of using 0.0.YYYY as the version layout for C++ standards is quite unusual. I understand that you are doing it for technical reasons (the patch number is the only entry that can contain four digits numbers), but I still think that a more intuitive solution is needed. One solution could be to use an encoding similar to your BOOST_PREDEF_MAKE_YYYY_MM_DD macro. Another solution could be to define an explicit macro for each supported standard. E.g. #if __cplusplus - 0 >= 199711L # define BOOST_LANGUAGE_CXX98 1 #endif Indentation of preprocessor directives ====================================== Old preprocessors will only recognize directives if these is a hash (#) in the first column of the line. The most portable way of indenting directives is therefore to put the hash sign in the first column and then put whitespaces between the hash sign and the rest of the directive. E.g. "# if" instead of " #if". Else if ======= You use the #elif directive in some files. Notice that older Borland preprocessors will fail with that directive. The portable way to do else-ifs is the more tedious alternative: #if MACRO #else # if MACRO2 # endif #endif Apropos else-if, in compiler/greenhills.h you are using the incorrect "#else if defined" construct. Credit ====== As the main author of the predef.sourceforge.net pages, it is pretty clear to me that you have extracted much information from those pages, so I think that a reference to those pages may be in order.